This invention relates to a shading correcting device which is used in a reading device having a solid-state image pickup element.
Solid-state image pickup elements are used extensively in reading and recording devices such as facsimile devices or other copying machines. The image pickup element reads the picture datum on an original and converts the read information into a series of electrical signals.
FIG. 1 shows one example of such a reading device. An original 2 is placed on a transparent platen 1 in such a manner that the surface of the original to be read faces downwardly. A fluorescent lamp 3 is provided below the platen 1 in such a manner that light therefrom is extended in the main scanning direction of the original 2. Light from the fluorescent lamp 3 is applied to the original 2, and light reflected from the original 2 is applied through a lens 4 to a solid-state image pickup element 5 which forms the optical image. The image pickup element is, for instance, a one-dimensional image pickup element which utilizes a CCD. As the original 2 is moved in the auxiliary scanning direction, the picture datum is read in a raster scan pattern.
With the above-described recording device, even when an original is uniform in density over a line (as in the case of a substantially blank original), the photo-electric conversion output of the solid-state image pickup element 5 is not uniform. One reason for this phenomena is that the light source is not uniform in luminance distribution. As shown in FIG. 2, when a fluorescent lamp 3 is used as the light source, the rays 6 therefrom are concentrated at the center of a reading line. Accordingly, the illuminance is highest at the center of the original, and it decreases towards either end of the original. Thus, the photo-electric conversion output is not uniform. Other reasons for the non-uniform photo-electric output are that the quantity of light at the periphery of the lens 4 is small due to the cosine biquadrate rule, and the solid-state image pickup element 5 is not uniform in sensitivity.
If the photo-electric conversion output of the solid-state image pickup element 5 is not uniform, a subsequent signal processing operation such as a binary-encoding of the analog picture signals is adversely affected, resulting in a reduction in the quality of picture. This phenomena will be described in greater detail with reference to FIG. 3. In the description to follow, it is assumed that an original's reading line has picture data 7 (black-and-white data) as shown in FIG. 3A. In this case, the solid-state image pickup element provides a non-uniform photo-electric conversion output 8 as shown in FIG. 3B. The output 8 is binary-encoded by comparison with a preset threshold level l.sub.1. In this case, in the central portion of a line, a signal level corresponding to black picture data (hereinafter referred to as a "black level", when applicable) may be binary-encoded erroneously as white picture data. In addition, in the vicinity of each end of a given picture line, a signal level corresponding to white picture data (hereinafter referred to as a "white level", when applicable) may be binary-encoded erroneously as black picture data. Accordingly, when a threshold level l.sub.1 is provided as shown in FIG. 3B, the generated binary-encoded picture signal 9 is considerably deteriorated when compared to the original picture data.
In order to prevent the picture data from being deteriorated in the binary-encoding operation, a shading correcting device is known in the art in which a threshold level is set by an A/D converter and a D/A converter. With reference to FIG. 4A, a white (blank) line is first read by the solid-state image pickup element so that a photo-electric conversion output (a shading waveform) 11 over a line is obtained. Then, the output 11 is converted into digital data by the A/D converter, and the digital data is stored in a memory. Thereafter (i.e., when the picture signals are actually read), the digital datum are converted into analog datum by the D/A converter. According to the analog datum, a threshold level l.sub.2 similar to the shading waveform 11 is set as shown in FIG. 4B, so that the picture signal 12 is subjected to binary-encoding. Accordingly, the white and black levels of the picture data are binary-encoded correctly, as a result of which a digital picture signal 13 high in quality can be obtained as shown in FIG. 4C.
The above-described conventional shading correcting device is disadvantageous in that it necessitates the use of a D/A converter for converting digital signals which are obtained through A/D conversion into analog signals.